i) Field of the Invention
This invention relates to an aqueous emulsion which is useful in the manufacture of water-resistant gypsum products; the invention also relates to a composition settable by hydration to form a water-resistant gypsum product and to a water-resistant gypsum panel or board; the invention also relates to a method for the manufacture of gypsum panel or board.
ii) Description of Prior Art
Gypsum is employed in a gypsum panel or board product known as wallboard which is widely used as a structural building panel.
Gypsum products are produced by mixing anhydrous calcium sulphate or calcium sulphate hemihydrate with water and allowing the mixture to hydrate or set as calcium sulphate dihydrate which is relatively hard.
Gypsum wallboard comprises a panel-like core of set gypsum sandwiched between a pair of paper liners which form the exposed outer surfaces of the wallboard. Fiberglass liners have also been used.
In many applications wallboard is exposed to water. A problem with set gypsum is that it absorbs water, and such absorption reduces the strength of the wallboard. There have been many proposals to render gypsum wallboard water-resistant or water-repellent. U.S. Pat. No. 3,935,021 describes some of these prior proposals and problems associated with them.
U.S. Pat. No. 3,935,021 describes a gypsum wallboard in which polyvinyl alcohol and a wax-asphalt emulsion are incorporated in the gypsum core. Wax-asphalt emulsions are widely employed in gypsum wallboard, however, certain disadvantages are inherent in their use.
In practice the wax-asphalt emulsion is not prepared at the site of use in wallboard manufacture, but is prepared by a different manufacturer at a different site and is shipped to the wallboard manufacturer.
Wax-asphalt emulsions have a tendency to separate on storage with time, forming a crust which floats on the surface of the emulsion in the storage tank. The crust, after a period of time, cannot be readily re-emulsified by mixing but only dispersed into smaller lumps or particles, and this can result in the production of black spots on the paper liners due to bleed out of the unemulsified asphalt from the set gypsum core. In addition, the separated wax-asphalt eventually needs to be disposed of, thereby causing environmental problems. The wax-asphalt emulsion has a black colour because of the asphalt.
Furthermore, as asphalt is a residue or by-product of petroleum refining, it is a relatively crude and variable material. As a result, wax-asphalt emulsions have not been consistent in their efficiencies to waterproof gypsum wallboard, thereby causing problems for the gypsum board manufacturers in determining usage levels required on a batch-to-batch basis.